This invention relates to a process for the manufacture of products from reinforced polyester resins and, more particularly, polyester resin systems which are reinforced with glass fibers. In accordance with the process, encapsulated reaction additives such as initiators remain functionally isolated in the blended molding materials until released by preselected process conditions. Products manufactured from reinforced polyester resins are widely used in automotive, appliance, and other industries.
In the processes now employed, polyester resin reactants are added to a large mixer along with the additives, such as promoters or accelerators, inhibitors, pigments, stearates, fillers, thermoplastic profiling compounds, and the initiator or catalyst. The material is intensively mixed with high energy shearing for perhaps 20 minutes and then it is made into sheet molding compound (SMC), thick molding compound (TMC) or bulk molding compound (BMC).
With SMC, the reinforcing glass fibers are of variable length and are introduced as the mixture of resin and additives is disposed between sheets of protective polyethylene film. Optionally, continuous lengths of glass fiber roving or mat may be disposed between sheets of compound, which in turn are covered, top and bottom, with protective polyethylene film. The film-protected SMC composite is passed between rollers which knead the composite in order to thoroughly mix and wet the glass fiber with resin reactants. The SMC is up to perhaps 1/4 inch thick. There are special high strength forms of SMC based upon particular fiberglass reinforcement characteristics. The TMC comes in sheet form like SMC, but may be up to several inches thick.
With BMC, short glass fibers 1/8 to 11/4 inches long are the reinforcement and are added at the time of mixing. BMC has a consistency similar to that of modeling clay and is extruded into logs or ropes, or pelletized, or may be used right out of the mixer.
The ready-to-mold resin and reinforcement composites in the form of SMC and BMC materials are made up intermittently in large batches in accordance with production demand. After mixing, they are stored under controlled conditions until they are used. Because the initiator is mixed into the compound, the compounds are partially cured and gelled, which increases their viscosity, and they continue to cure slowly in storage. After one or two months in storage, they become too cured or viscous to use and must be discarded. They thus have limited shelf life, depending upon storage conditions.
In the course of the mixing, the temperature increases because of the energy put into the compound and because the reaction is exothermic. The intensity and length of the mixing process must be restricted to avoid excessive premature cure of the compound. Water jacket cooling techniques may be employed, but the mixing operation remains an art with variation of resin materials and initiators, as well as the possible use of promoters or accelerators and inhibitors. Often, the mixing process is simply terminated just prior to a critical temperature (e.g., 32.degree. C.).
One problem with SMC and BMC materials is that because of the restricted conditions under which they are mixed, it often happens that the initiator and fillers are not completely distributed throughout the mixture. This frequently occurs if the temperature of the mixture increases too much and the mixing operation has to be cut short before the additives are completely mixed in.
SMC is, for the most part, molded in matched metal die compression molds. It usually is about 24 inches wide and is weighed and cut into suitable lengths for insertion into the molds. BMC is likewise molded in compression molds. Pieces of BMC are cut off by weight and placed into molds. Charges can weigh as much as 30 pounds or more. BMC can be preheated in a screw and injected into the mold. It can also be injected-molded with a plunger. SMC, TMC, and BMC materials can also be molded in transfer molds. At this writing, most production uses either SMC or BMC compounds. The use of TMC is limited.
It is desirable for cost purposes to minimize the molding cycle or time, which tends to increase with the weight of the charge to the mold. To that end, increased amounts of initiators are used in combination with inhibitors, which act as free radical traps and tend to prevent premature initiation of polymerication. Preferably, the effect of the initiator is depressed during the storage of the compounds to improve shelf life, as well as during the mold filling process. However, at the desired point of cure, the initiators should cause rapid cure at high temperatures. Heretofore, these ideal conditions have been sought through the use of combinations of initiators and inhibitors, as well as promoters or accelerators, which tend to lower the decomposition temperature of the initiator. Combinations of these reaction additives involve trade-offs in the physical properties of the cured resin. Further reference is made to U.S. Pat. No. 2,632,751, columns 1 and 2.
For the products molded from SMC and BMC materials, there have always been problems in filling the molds completely and in obtaining suitable surface finish of the molded parts, even though inhibitors are used to delay the curing reaction and viscosity increases. From 10-20% of the products so molded have to be hand-finished, which is expensive and time-consuming. Even with hand-finishing, the scrap rate for these molded products may be in the order of 50%. The molded products are usually painted, but they have to be washed and cleaned before they can be painted. In both the cleaning and painting steps, the products are heated back up to temperatures which approach those at which they were molded. Most products are between 85% and 90% cured when they come out of the compression mold. The heating from the washing and painting increases the degree of cure, but also relieves stresses in the parts, causing warpage and distortion.
Curable resin compositions containing encapsulated catalysts are known. U.S. Pat. No. 3,860,565 teaches the encapsulation of the catalysts for isocyanate resins and identifies a number of other patents relating to curable resin systems with encapsulated catalysts. So far as I know, however, no one has ever taught or suggested the encapsulation of the initiators or catalysts in polyester resin systems. These initiators are usually organic peroxides in the form of volatile liquids. Such initiators are toxic and difficult to handle.
The processes for the micro-encapsulation of materials as well known, and have been well known ever since the end of World War II. Reference is made to "Microcapsule Processing and Technology," Asaji Kondo, edited and revised by J. Wade Van Valkenburg, Marcel Decker, Inc., New York, N.Y. (1979), and "Capsule Technology and Microencapsulation," Noyes Data Corp., Park Ridge, N.J. (1972).